candidate

# File lib/optparse.rb, line 1593
  def candidate(word)
    list = []
    case word
    when /\A--/
      word, arg = word.split(/=/, 2)
      argpat = Completion.regexp(arg, false) if arg and !arg.empty?
      long = true
    when /\A-(!-)/
      short = true
    when /\A-/
      long = short = true
    end
    pat = Completion.regexp(word, true)
    visit(:each_option) do |opt|
      next unless Switch === opt
      opts = (long ? opt.long : []) + (short ? opt.short : [])
      opts = Completion.candidate(word, true, pat, &opts.method(:each)).map(&:first) if pat
      if /\A=/ =~ opt.arg
        opts.map! {|sw| sw + "="}
        if arg and CompletingHash === opt.pattern
          if opts = opt.pattern.candidate(arg, false, argpat)
            opts.map!(&:last)
          end
        end
      end
      list.concat(opts)
    end
    list
  end